The myofibroblast is an influential stromal cell of the gastrointestinal (GI) tract that regulates many important processes ranging from cell proliferation and differentiation along the crypt-villous axis, mucosal repair, fibrosis, and the development of cancer [7-9]. The signaling pathways that regulate myofibroblast function have been studied in vitro [10, 11]. However, the actual physiologic impact of these signaling events on the overlying epithelium remains largely speculative due to limitations with existing co-culture and animal models. In vitro co-culture models lack the normal bowel wall architecture and surrounding cell populations that are essential elements of the GI microenvironment [9]. Animal models that utilize conditional gene targeting are neither organ nor cell type-specific since the myofibroblast lacks a unique cell marker [12, 13]. Current research efforts are limited by a paucity of in vivo models to effectively study how the myofibroblast regulates epithelial cell function under normal and pathologic conditions. This proposal involves the development of a novel technique utilizing murine colonoscopy that will allow for the real-time in vivo study of myofibroblast-epithelial cell interactions. The technique first involves the isolation of primary myofibroblasts from mouse colon tissue and subsequent growth in cell culture. Primary myofibroblasts will then be re-implanted endoscopically by injection into the colon wall of an immune-competent, syngeneic mouse. The technique allows for targeted manipulation of a subpopulation of myofibroblasts in a segment of mouse colon that can be easily re-identified, serially evaluated, and directly compared to adjacent tissue under the same experimental conditions. This technique can be applied to any existing mouse model to study stromal-epithelial interactions in the distal colon. The proposal aims to 1) demonstrate that myofibroblasts can be successfully and reproducibly implanted in the colon wall and can maintain viability in live, immune-competent, syngeneic mice, and as a proof of principle, that 2) this model can be used to study stromal-epithelial cell interactions, specifically in the context of an animal model of injury-repair.